Since a few years it is proposed to use liquid crystal displays (LCD's) as a primary image source in projection display systems. The use of LCD's in projection displays may be seen in allowed U.S. patent application Ser. No. 292,618, filed Dec. 29, 1988, of the inventor Jill Goldenberg and others.
In an LC-projection system of the kind disclosed in beforementioned applications the beam of light produced by a white light source, such as a tungsten-halogen lamp, is separated by a pair of dichroic mirrors in three subbeams, each one of which contains light of one of the primary colors, red, green and blue. Each of the subbeams is made incident upon a tunable birefringent light valve such as a transmission LCD. The three light valves modulate the three channels to create the red, green and blue portions of a TV picture. The three color portions are then recombined by way of a second set of dichroic mirrors. The recombined light is projected via a projection lens system onto a projection screen.
In the known system the light rays incident upon each picture element (pixel) of a light valve passes through or reflects from only part of each of the dichroic mirrors. For different pixels different although overlapping portions of the mirrors are used for separtion and recombination of the subbeams. The average angle at which the light ray pass through or reflect the dichroic mirrors varies in dependence of the position of the pixel in the light valve. As dichroic mirrors have a cut-off wavelength, or a transmission/reflection characteristic, which is angle dependent, this means that different pixels of the same light valve receive and consequently transmit light of a slightly different color. The cut-off wavelength is defined as the wavelength for which 50% of the light is transmitted or reflected. The angle dependent shift in the cut-off wavelength results in a projected color picture exhibiting a monotonic color change from one side of the projected image to the other.